The metabolism in an organism is reduced to a network of
substances. The resulting degree-distribution is power law
like with an exponent about 2.2. 
In order to understand this, we use information theory to
obtain a null-model defined by assigning equal probabilities
to what is assumed to be the fundamental network
possibilities. A stochastic variant of variational calculus
is used to obtain the corresponding degree distribution for
the null-model. The striking agreement implies that the null
model catches the overall feature of the metabolic network.
Using the network structure measures like clustering and
assortativity, a small difference is identified as the only
sign of any possible evolutionary pressure. However, this
difference is only manifested in a slight difference in the
degree distribution and seemingly not in any particular
network design.